In conventional refrigerators and air conditioners for cooling and warming air configured of compressors, radiators, flow control valves, and evaporators, which are connected by coolant pipes and configured in such a way that a hydrofluorocarbon coolant (hereinafter referred to as an HFC coolant) circulates, the global warming potential of the HFC coolant is relatively large, which cause evil effects of the global warming.
Refrigerators and air conditioners for cooling and warming are now developed using a hydrocarbon coolant (hereinafter referred to as an HC coolant) such as propane, ammonia, and carbon dioxide, whose global warming potential values are lower than that of chlorofluorocarbon. When the HC coolant or ammonia is used, because these coolants are flammable, measures not to ignite themselves are needed; therefore, the usage is limited by the law. Although carbon dioxide is nonflammable, a problem is included in which the coefficient of performance (hereinafter referred to as the COP) deteriorates.
In a case of an air conditioner as an example of a refrigerator using carbon dioxide as a coolant, the reason is explained why the COP deteriorates when carbon dioxide is used as the coolant. An air conditioner has cooling/warming rate conditions that define atmospheric temperatures. In a cooling operation, when dry-bulb temperature is 35; degrees outside a room, the dry-bulb temperature is 27 degrees and wet-bulb temperature is 19 degrees inside the room. In a warming operation, when the dry-bulb temperature is 7 degrees and the wet-bulb temperature is 6 degrees outside the room, the dry-bulb temperature is 20 degrees inside the room. In a case in which carbon dioxide is used as the coolant, the COP in a cooling rate condition especially deteriorates under the outdoor temperature being relatively high. This phenomenon is caused by the coolant temperature increasing up to not lower than 35 degrees at the exit of a heat exchanger placed outside the room, because the dry-bulb temperature outside the room is 35 degrees. When carbon dioxide expands from the super critical state, a region in which the specific heat is relatively large exists in approximately from 10 to 60 degrees; however, in a state in which the dry-bulb temperature outside the room is 35 degrees, because the entire of the region in which the specific heat is relatively large cannot be used, the energy consumption efficiency decreases. On the other hand, when the HFC coolant or the HC coolant is used, heat exchange is possible in which the coolant vapor can be wholly changed into the coolant liquid under the cooling rate condition; therefore, the COP is more improved than that in the case of carbon dioxide.
A conventional air conditioner using carbon dioxide as a coolant is disclosed, in which a coolant cooling means composed of a cooling heat-exchanger, using a low-temperature heat source including water, ice-water, and seawater, is provided, and by sequentially connecting, using coolant pipes, a compressor, a radiator, the coolant cooling means, a flow control valve, and an evaporator, the coolant is circulated. This objective is to improve the COP by decreasing, using the coolant cooling means, the coolant temperature at the entrance of the flow control valve (for example, referring to Patent Document 1).
As a cooling means for cooling the coolant at the entrance of the flow control valve, some power is needed as the cooling means, when water or seawater, etc. in which the power is not needed cannot be used. This power is increased corresponding to the cooling ability of the cooling means. Therefore, considering the sum of the power needed for the compressor and the cooling means that are provided in the air conditioner, overcooling causes the increase of the power needed for the cooling means; consequently, the COP deteriorates. When the cooling is insufficient, the power needed for the compressor of the air conditioner increases; as a result, the COP deteriorates.
[Patent Document 1] Japanese Laid-Open Patent Publication 54,617/1998.